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Journal of Neuroscience, Vol 10, 73-90, Copyright © 1990 by Society for Neuroscience
Isolation, biochemical characterization and ultrastructural analysis of the limbic system-associated membrane protein (LAMP), a protein expressed by neurons comprising functional neural circuits
A Zacco, V Cooper, PD Chantler, S Fisher-Hyland, HL Horton and P Levitt
Department of Anatomy, Medical College of Pennsylvania, Philadelpha 19129.
The limbic system-associated membrane protein (LAMP) is a cell surface
glycoprotein expressed by cortical and subcortical regions of the mammalian
CNS that comprise or receive direct projections from limbic system
structures. The early and restricted expression of LAMP has led to its
postulated role in neural development. Purification and biochemical
characterization of LAMP was performed in order to ascertain its
relationship to other, well-defined cell surface proteins in the nervous
system. Subcellular fractionation, immunoaffinity chromatography, and
Western blots of rodent and bovine hippocampus revealed that LAMP is an
integral membrane protein with a molecular mass of 64-68 kDa and a pI of
5.2-5.5. Deglycosylation of LAMP indicates that it contains N-linked high
mannose or hybrid sugars and a minor amount of sialic acid. The LAMP
protein exhibits an identical molecular mass in developing hippocampus and
in several different brain regions in the adult. No cross-reactivity was
obtained using the monoclonal antibody that recognizes the HNK-1
carbohydrate epitope, a complex sulfated moiety expressed on members of a
large family of glycoproteins. Immunocytochemical analysis at the
ultrastructural level reveals that LAMP immunoreactivity is exhibited by
neurons in a stereotyped pattern throughout limbic system areas. Glial
cells are not immunoreactive. In the adult, LAMP-immunoreactive membrane
patches are present exclusively postsynaptically on neuronal somata and
dendrites. Myelinated and unmyelinated axons are not stained in any brain
region examined. Analysis of LAMP expression in the developing CNS during
synaptogenesis demonstrates that LAMP is located on growing axons and both
pre- and postsynaptically at forming terminal complexes. Double- labeling
studies of the hippocampal neurons grown in vitro reveal that the LAMP
epitope is extracellular and is expressed on neurofilament- and
microtubule-associated protein 2-positive neurites. Cells expressing glial
fibrillary acidic protein are not LAMP-immunoreactive. These results
demonstrate that in the adult brain, LAMP is expressed almost exclusively
by the postsynaptic (target) elements in limbic circuits, but that during
development, all components of the surface of the growing neuron contain
LAMP. The stereotyped anatomical pattern of expression of LAMP in the
developing and mature brain and its biochemical characteristics suggest
that LAMP is a unique, system- associated membrane glycoprotein that is
distinct from previously identified, developmentally important cell surface
proteins.
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